The invention relates to a smoke sensor which detects smoke, and a monitor control system.
Conventionally, as a light scattering smoke sensor, a smoke sensor is disclosed in, for example, Japanese Patent Unexamined Publication No. Sho. 51-15487. In the disclosed smoke sensor, a light emitting diode is driven by a circuit which generates plus and minus rectangular waves, and two kinds of light of different wavelengths .lambda..sub.1 and .lambda..sub.2 are temporally alternately emitted by the light emitting diode in response to the plus and minus rectangular waves. A single light receiving device receives scattered light which is produced by smoke or the like from the two kinds of light of different wavelengths .lambda..sub.1 and .lambda..sub.2 emitted by the light emitting diode. A ratio (two-wavelength ratio) of scattered light outputs of the two different wavelengths .lambda..sub.1 and .lambda..sub.2 is obtained. It is determined whether the two-wavelength ratio is in a predetermined range or not. If the ratio is in the range, an alarm is activated.
In the smoke sensor, it is intended that the kind (characteristic) of smoke is judged (for example, only smoke in which the particle diameter is in a specific range is detected) by determining whether the two-wavelength ratio is in the predetermined range or not. In other words, the smoke sensor is developed in order to eliminate an influence due to dust, steam, or the like which is not a fire cause, and detect only smoke which is produced by a fire cause.
However, in a smoke sensor configured so as to temporally alternately receive scattered light of two different wavelengths .lambda..sub.1 and .lambda..sub.2 as described above, the timing of the detection of scattered light of the wavelength .lambda..sub.1 is not identical with (the same time as) that of scattered light of wavelength .lambda..sub.2. Therefore, a ratio y/g of the scattered light output (light intensity output) y of the wavelength .lambda..sub.1 to the scattered light output (light intensity output) g of the wavelength .lambda..sub.2, i.e., a two-wavelength ratio contains many errors, and hence accurate smoke detection is limited.